1. Field of the Invention
The present invention relates to an optical communication system, and more particularly to monitoring the states of optical elements in an optical amplifier used in an optical communication system and informing degradation or failure of the optical elements.
2. Description of Related Art
Recently, an optical communication system has been developed for necessity of transmission of a large amount of data. In the optical communication system, since an optical signal attenuated on the way of transmission, it is necessary to be amplified by an optical amplifier provided at an optical relay point or an exchange station. If the optical amplifier fails, the optical signal is not amplified so that correct communication cannot be achieved. Therefore, information of degradation or failure of the optical elements are required for correct amplification of the optical signal.
A conventional amplifier unit will be described below with reference to FIG. 1. Referring to FIG. 1, the optical amplifier unit includes a rare earth element doped optical fiber 201 which is connected to normal or undoped optical fibers 211 and 212. Optical filters 205 are provided between the optical fibers 211 and 201 and between the optical fibers 201 and 212. As described above, the optical fiber 201 is doped with rare earth element such as neodymium (Nd) and erbium (Er). The optical characteristic of optical fiber 201 is shown in the Japanese Laid Open Patent Disclosure (JP-A-hei3-7917). In this reference, such an optical fiber is used to separate a desired optical signal from a transmitted signal. A light emitting element 202 such as a semiconductor laser emits light in accordance with a drive current from a drive circuit 220 and the emitted light is amplified by an amplifier 203 such as another semiconductor laser. The amplified light by the amplifier 203 is supplied and coupled to the optical fiber 201 by an optical coupler 204 via a filter 218. Thus, the optical signal input from the optical fiber 211 to the optical fiber 201 is amplified. Such an optical amplifier is disclosed in, for example, the Japanese Laid Open Patent Disclosure (JP-A-Hei4-1614). The filter 218 is provided to prevent the light from being supplied from the optical fiber 201 to the optical semiconductor laser 203. A signal indicative of the level of drive current is supplied from the drive circuit 220 to a monitor circuit 224. The monitor circuit 224 determines the states of optical elements based on the signal from the drive circuit 220. In this case, when the drive current is increased 1.5 times more than an initial drive current, the monitor circuit 224 determines that any of the optical elements in the amplifier unit failed.
In this manner, in the conventional optical element monitoring system, failure of the optical element is monitored based on only the incremental amount of drive current. Therefore, whether output drop is caused due to the failure of optically active light emitting element or the failure of optical passive elements such as the filter 218 and optical coupler 204 cannot be determined.